Both Bluetooth and WLAN radio devices, such as those used in, for example, handheld wireless terminals, generally operate in the 2.4 GHz (2.4000-2.4835 GHz) Industrial, Scientific, and Medical (ISM) unlicensed band. Other radio devices, such as those used in cordless phones, may also operate in the ISM unlicensed band. While the ISM band provides a suitable low-cost solution for many of short-range wireless applications, it may also have some drawbacks when multiple users operate simultaneously. For example, because of the limited bandwidth, spectrum sharing may be necessary to accommodate multiple users and/or multiple different types of communication protocols. Multiple active users may also result in significant interference between operating devices. Moreover, in some instances, other devices such as microwave ovens may also operate in this frequency spectrum and may produce significant interference or blocking signals that may affect Bluetooth and/or WLAN transmissions.
When operating a Bluetooth radio and a WLAN radio in, for example, a wireless device, at least two different types of interference effects may occur. First, when an interfering signal is present in a transmission medium along with the signal-of-interest, a low signal-to-noise-plus-interference ratio (SINR) may result. In this instance, for example, a Bluetooth signal may interfere with a WLAN signal or a WLAN signal may interfere with a Bluetooth signal. The second effect may occur when the Bluetooth and WLAN radio devices are collocated, that is, when they are located in close proximity to each other so that there is a small radio frequency (RF) path loss between their corresponding radio front-end receivers. In this instance, the isolation between the Bluetooth radio front-end and the WLAN radio front-end may be as low as 10 dB, for example. As a result, one radio may desensitize the front-end of the other radio upon transmission. Moreover, since Bluetooth employs transmit power control, the collocated Bluetooth radio may step up its power level when the signal-to-noise ratio (SNR) on the Bluetooth link is low, effectively compromising the front-end isolation between radio devices even further. Low noise amplifiers (LNAs) in the radio front-ends may not be preceded by a channel selection filter and may be easily saturated by the signals in the ISM band, such as those from collocated transmissions. The saturation may result in a reduction in sensitivity or desensitization of the receiver portion of a radio front-end, which may reduce the radio front-end's ability to detect and demodulate the desired signal.
Different techniques have been developed to address the low isolation problem that occurs between collocated Bluetooth and WLAN radio devices in coexistent operation. These techniques may take advantage of either frequency and/or time orthogonality mechanisms to reduce interference between collocated radio devices. Moreover, these techniques may result from so-called collaborative or non-collaborative mechanisms in Bluetooth and WLAN radio devices, where collaboration refers to any direct communication between the protocols. For example, Bluetooth technology utilizes Adaptive Frequency Hopping (AFH) as a frequency division multiplexing (FDM) technique that minimizes channel interference. In AFH, the physical channel is characterized by a pseudo-random hopping, at a rate of 1600 hops-per-second, between 79.1 MHz channels in the Bluetooth piconet. AFH provides a non-collaborative mechanism that may be utilized by a Bluetooth device to avoid frequencies occupied by a spread spectrum system such as a WLAN system. In some instances, the Bluetooth radio may be enabled to modify its hopping pattern based on, for example, frequencies in the ISM spectrum that are not being occupied by other users.
Even when frequency division multiplexing techniques are applied, significant interference may still occur because a strong signal in a separate channel may still act as a blocking signal and may desensitize the radio front-end receiver, that is, increase the receiver's noise floor to the point that the received signal may not be clearly detected. For example, a collocated WLAN radio front-end transmitter generating a 15 dBm signal acts as a strong interferer or blocker to a collocated Bluetooth radio device receiver when the isolation between radio devices is only 10 dB. Similarly, when a Bluetooth radio device is transmitting and a WLAN radio device is receiving, particularly when the Bluetooth radio front-end transmitter is operating as a 20 dBm Class 1 type, the WLAN radio device receiver may be desensitized by the Bluetooth transmission as the isolation between radios is reduced.
Oscillators may be utilized in wireless receivers and transmitters to provide frequency conversion, and to provide sinusoidal sources for modulation. The oscillators may operate over frequencies ranging from several kilohertz to many gigahertz, and may be tunable over a set frequency range. A typical oscillator may utilize a transistor with a LC network to control the frequency of oscillation. The frequency of oscillation may be tuned by adjusting the values of the LC resonator. A crystal controlled oscillator (XCO) may be enabled to provide an accurate output frequency, if the crystal is in a temperature controlled environment. A phase locked loop (PLL) may utilize a feedback control circuit and an accurate reference source such as a crystal controlled oscillator to provide an output that may be tunable with a high accuracy. Phase locked loops and other circuits that provide accurate and tunable frequency outputs may be referred to as frequency synthesizers.
Phase noise is a measure of the sharpness of the frequency domain spectrum of an oscillator, and may be critical for many modern wireless systems as it may severely degrade the performance of a wireless system. The phase noise may add to the noise level of the receiver, and a noisy local oscillator may lead to down conversion of undesired nearby signals. This may limit the selectivity of the receiver and the proximity of spacing adjacent channels in a wireless communication system.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.